tree-optimization/94103 avoid CSE of loads with padding

VN currently replaces a load of a 16 byte entity 128 bits of precision
(TImode) with the result of a load of a 16 byte entity with 80 bits of
mode precision (XFmode).  That will go downhill since if the padding
bits are not actually filled with memory contents those bits are
missing.

2020-03-12  Richard Biener  <[email protected]>

	PR tree-optimization/94103
	* tree-ssa-sccvn.c (visit_reference_op_load): Avoid type
	punning when the mode precision is not sufficient.

	* gcc.target/i386/pr94103.c: New testcase.

gcc/ChangeLog

@@ -1,3 +1,9 @@
+2020-03-12  Richard Biener  <[email protected]>
+
+	PR tree-optimization/94103
+	* tree-ssa-sccvn.c (visit_reference_op_load): Avoid type
+	punning when the mode precision is not sufficient.
+
 2020-03-12  H.J. Lu  <[email protected]>
 
 	PR target/89229

gcc/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-03-12  Richard Biener  <[email protected]>
+
+	PR tree-optimization/94103
+	* gcc.target/i386/pr94103.c: New testcase.
+
 2020-03-12  Tobias Burnus  <[email protected]>
 
 	PR middle-end/94120

gcc/testsuite/gcc.target/i386/pr94103.c

@@ -0,0 +1,17 @@
+/* { dg-do run { target lp64 } } */
+/* { dg-options "-O3" } */
+
+int main()
+{
+  long double x;
+  unsigned long u[2] = {0xEEEEEEEEEEEEEEEEUL, 0xEEEEEEEEEEEEEEEEUL};
+  __builtin_memcpy(&x, &u, sizeof x);
+  __builtin_memcpy(&u, &x, sizeof u);
+  ++*(unsigned char *)&x;
+  (void)-x;
+  __builtin_memcpy(&u, &x, sizeof u);
+  if (u[1] != 0xEEEEEEEEEEEEEEEEUL
+      || u[0] != 0xEEEEEEEEEEEEEEEFUL)
+    __builtin_abort ();
+  return 0;
+}

gcc/tree-ssa-sccvn.c

@@ -4899,13 +4899,22 @@ visit_reference_op_load (tree lhs, tree op, gimple *stmt)
   if (result
       && !useless_type_conversion_p (TREE_TYPE (result), TREE_TYPE (op)))
     {
-      /* We will be setting the value number of lhs to the value number
-	 of VIEW_CONVERT_EXPR <TREE_TYPE (result)> (result).
-	 So first simplify and lookup this expression to see if it
-	 is already available.  */
-      gimple_match_op res_op (gimple_match_cond::UNCOND,
-			      VIEW_CONVERT_EXPR, TREE_TYPE (op), result);
-      result = vn_nary_build_or_lookup (&res_op);
+      /* Avoid the type punning in case the result mode has padding where
+	 the op we lookup has not.  */
+      if (maybe_lt (GET_MODE_PRECISION (TYPE_MODE (TREE_TYPE (result))),
+		    GET_MODE_PRECISION (TYPE_MODE (TREE_TYPE (op)))))
+	result = NULL_TREE;
+      else
+	{
+	  /* We will be setting the value number of lhs to the value number
+	     of VIEW_CONVERT_EXPR <TREE_TYPE (result)> (result).
+	     So first simplify and lookup this expression to see if it
+	     is already available.  */
+	  gimple_match_op res_op (gimple_match_cond::UNCOND,
+				  VIEW_CONVERT_EXPR, TREE_TYPE (op), result);
+	  result = vn_nary_build_or_lookup (&res_op);
+	}
+
       /* When building the conversion fails avoid inserting the reference
          again.  */
       if (!result)